The present invention generally relates to a method for determining an endpoint during chemical-mechanical polishing of a semiconductor wafer, and more particularly to a method for determining an endpoint on the basis of a semiconductor wafer with an additional layer. The present invention further generally relates to a semiconductor wafer for use in a chemical-mechanical polishing process, and more particularly to a semiconductor wafer having an additional layer on basis of which an endpoint is determined.
Chemical-mechanical polishing or planarization (xe2x80x9cCMPxe2x80x9d) processes are for example used in the production of microelectronic devices for forming a flat surface on semiconductor wafers.
FIG. 1 schematically illustrates a conventional polishing device 10. The polishing device comprises a polishing platen with a polishing pad 14 thereon. A wafer 16 is carried by a wafer carrier 18. The polishing platen 12 is carried by a drive assembly 20.
During operation of the conventional polishing apparatus 10 according to FIG. 1, the wafer 16 is positioned on the polishing pad 14 on which a polishing slurry is provided. Due to a rotational and/or translational movement of the wafer carrier 18 and/or the drive assembly 20, the wafer 16 and the polishing pad 14 are moved relative to one another. Due to the moving of the face of the wafer 16 across the surface of the polishing pad 14, the polishing pad 14 and the slurry remove material from the wafer 16.
It is extremely important that by chemical-mechanical polishing thicknesses of the polished layers are achieved that are within a certain range. Further, the surface of the wafer has to be uniform and planar. The correct thicknesses are important, since they influence the functional behavior of the wafer in operation. A planar surface is for example desired, when lithographic techniques are used in order to form patterns on the wafer. Such patterns can only be formed properly when the light for the lithographic process can be tightly focussed within a certain range. However, a reliable focussing is only possible with a planar surface.
Several techniques in order to achieve a correct xe2x80x9cendpointingxe2x80x9d for a polishing process have been proposed. For example, it has been suggested to use the change of the optical appearance of the wafer surface when a particular layer has been removed. Also a change in frictional behavior has been suggested to be used as an indication of an endpoint. However, there are general problems related to all of these techniques. For example, the monitoring of the optical appearance is difficult, since only an average value of the appearance of the wafer surface may be obtained, for example by a scanning operation; such an average value is not satisfying in all cases, particular when the structures on the wafer surface become smaller and smaller. Further, it may happen, that the optical appearance does not change at all, although a layer has been removed. Similar problems are related to a frictional determination of an endpoint.
Another endpointing method is disclosed in U.S. Pat. No. 6,057,602. It has been suggested to deposit an additional layer on the desired endpoint thickness, and, thereafter, to position the material to be polished. The additional layer has an extremely low polishing rate, so that the polishing xe2x80x9cautomaticallyxe2x80x9d stops when the additional layer is reached. However, it is a drawback of such method that a non-planar structure of the additional layer, for example due to a topography, is probably not sufficiently planarized. This leads to the problems discussed above when lithographic methods are applied to the wafer.
As mentioned above, conventional endpointing methods in CMP processes detect the transition from a material that has just been removed to a material which is uncovered by the polishing. Thus, the conventional methods are not applicable, when there is no material transition, for example in an interlayer dielectric (ILD) oxide polish. The present invention seeks to solve the above mentioned problems and to provide a method and a semiconductor wafer that allow reliable endpointing and accurate polishing results.